1. Field of the Invention
The present invention relates to a reflection-type Liquid-Crystal Display (LCD) device. More particularly, the invention relates to a reflector for a reflection-type LCD device that forms its light source by reflecting external incident light, and a LCD device using the reflector.
2. Description of the Related Art
Conventionally, reflection-type LCD devices are known well. Reflection-type LCD devices have an advantage that the power consumption is low and the body is thin and light, compared with transmission-type LCD devices. This is because external light can be used for their light source by reflecting it with a built-in reflector, which eliminates the backlight. Thus, reflection-type LCD devices are mainly used for portable electronic terminals such as portable telephones.
The reflection-type LCD device comprises various modes or types, such as the twisted-nematic (TN) mode, the single polarizer type, the super twisted-nematic (STN) mode, the guest-host mode, the polymer-distributed liquid crystal (PDLC) type, and the cholesteric phase type. The basic configuration of the reflection-type LCD device comprises a liquid crystal layer, switching elements for driving the liquid crystal in the layer, and a reflector plate provided inside or outside the LCD cell.
With the LCD devices of these modes or types, the active-matrix addressing method using Thin-Film Transistors (TFTs) or Metal/Insulator/Metal (MIM) diodes as the switching elements is employed along with the reflector. This is because it generates high resolution and high quality of image.
An example of the reflection-type LCD devices is as follows:
An organic dielectric film is formed on a plate and then, the film is patterned by photolithography and etching processes, forming isolated protrusions on the surface of the plate. The protrusions are formed by the remaining organic film. An interlayer dielectric film is formed on the plate to cover the protrusions, making protrusions on the surface of the interlayer dielectric film, in other words, making a bumpy surface of the interlayer dielectric film. Thus, a reflector plate having protrusions on its surface is obtained.
The Japanese Patent No. 2825713 issued on Sep. 11, 1998, discloses another example of the reflection-type LCD devices. In this example, an organic dielectric film is formed over a plate and then, the film is patterned by photolithography and etching processes, forming protrusions on the surface of the film. Thereafter, a patterned metal film serving as a reflector electrode is formed on the dielectric film to cover the protrusions. Thus, a substrate with the reflector electrode whose surface is smoothly roughened is formed.
FIG. 1 is a plan view showing a prior-art plate-shaped reflector 101, in which protrusions 102 are formed on the surface of the reflector 101. As shown in FIG. 1, all the protrusions 102 are circular in plan shape and arranged to be isolated front each other.
The prior-art reflector 101 has a purpose of reflecting incident light while diffusing the same at a certain extent and therefore, the reflected light has large dispersion. Thus, the incident light is reflected in such a way that the reflected light is diffused within a conical region approximately uniformly, as shown in FIG. 2.
FIG. 2 shows the relationship between the incident light and the reflected light by the prior-art reflector plate 101. As seen from FIG. 2, the incident light LIN (e.g., a fluorescent lamp or the sunlight) is irradiated to the plate 101 in the viewing direction of a viewer. The light LIN irradiated is reflected by the plate 101, forming reflected light beams LREF. The reflected beams LREF are approximately uniformly diffused by the reflector 101. Accordingly, the prior-art reflector 101 with the circular protrusions 102 has the following disadvantages.
First, when the prior-art reflector 101 is located in a situation (e.g., in a room) where direct light is dominant and indirect light is weak, part of the incident light LIN that propagates in a specific direction is not reflected efficiently toward the viewer. Here, the “direct light” is strong light emitted from a light source, such as a fluorescent lamp, and directly irradiated to the reflector 101 without reflection. The “indirect light” is light emitted from a light source, such as a fluorescent lamp, and indirectly irradiated to the reflector 101 after reflection at some positions (e.g., a wall). This means that the incident light LaIN is not utilized effectively. As a result, the intensity of the reflected right LREF toward the viewer decreases resulting in the viewer's feeling that images on the displaying screen is dark or is not sufficiently light.
Second, if the protrusions 102 of the reflector 101 have a specific geometric shape or arrangement pattern, there is a possibility that the color tone of images on the screen changes outstandingly according to the angle with respect to the LCD panel, the incident light LIN, and the position of the viewer. This is due to the optical interference caused by the difference between the optical path lengths of the beams of the reflected light LREF (i.e., which positions of the roughening pattern for the reflector 101 the incident light beam is reflected at). As a result, there is a possibility that the displaying performance of the LCD device of this type degrades.